Ink printing devices, such as ink-jet printers or offset printing presses, are known to incorporate curing lamps in order to cure known radiation curable inks, for example an ultra-violet (UV) curing lamp to cure a UV curable ink.
In such ink printing devices, a radiation curable ink is printed onto a substrate, such as paper, and then is cured by a curing lamp. The curing lamp cures substantially all of the ink by shining directly onto it. The substrate is generally moved throughout the device, from the location of the print head to the location of the curing lamp, by one or more rollers, belts, or the like.
Unfortunately, between when the ink is deposited by the print head and when it is cured by the curing lamp, the uncured ink can bleed into the substrate. For example, a liquid or molten uncured ink can bleed into the fibers of a paper substrate and can become at least partially visible from the backside of the substrate. This problem is known in the art as showthrough or strike-through, and is generally known to exist for any type of liquid ink deposited on a porous substrate. This issue is more pronounced in inks of low viscosity, such as ink jet inks, while higher viscosity inks such as litho inks are less susceptible to this problem. Specifically, showthrough is a measure of how colorized an ink makes the backside of the substrate.
A drawback of backside showthrough is the inability to do duplex printing. Particularly, when the ink wicks towards the opposite side of the paper, two-sided printing would not be possible, because the ink that shows-through to the opposite side could ruin the second print, or degrade the quality of print on both sides of the paper. Complete passage of the ink through the paper is not necessary for show through to be noticeable, even a small distance of travel into the paper may evoke a detectable difference compared to an ink that remains entirely on the surface.
The problem of showthrough is conventionally addressed in one or more of several known ways. First, showthrough can be minimized by controlling physical properties of the ink by, for example, controlling its viscosity as disclosed in U.S. Pat. No. 6,258,873, or by controlling its drying time as discussed in U.S. Pat. No. 6,428,159. Showthrough may also be minimized by coating the substrate with various polymers, such as is disclosed in U.S. Pat. No. 6,283,589. Known methods of varying ink composition and substrate coatings are the most widely used approaches to minimizing showthrough.
However, these approaches suffer from several disadvantages. For example, the ink composition and the substrate coating generally must be chemically compatible in order for showthough to be minimized. Specifically, ink composition properties such as drying time, viscosity, surface energy and polarity must be specifically tailored to match certain substrate coating properties such as porosity, ionic charge and hydrophobicity in order to result in decreased showthrough. In this way, either the ink or the surface coating often must be reformulated in order to work with the other, a situation that can preclude using other desired combinations of ink and substrate stock.
Other methods of minimizing showthrough are known. For instance, showthrough may be controlled by subjecting the printed image to fusing by applying to the image a fusing member at an elevated temperature, as disclosed in U.S. Pat. No. 7,202,883. Finally, commonly assigned U.S. Pat. No. 6,428,159 describes an ink printing apparatus that prevents showthrough by including a drying system that allows for rapidly evaporating water from an ink, while the ink is still resident on the paper surface. However, these approaches to dealing with showthrough suffer from disadvantages of requiring complicated machinery and being energy intensive, and thus lacking wide commercial viability. Furthermore, these approaches are not particularly suited to radiation curable inks, only for thermo-curable and evaporative inks respectively.
Therefore, there is a need for an apparatus and method for efficiently minimizing showthrough of a radiation curable ink on a substrate, which has wide applicability to various radiation curable inks and substrates.